In order to form a plurality of strips from a single, relatively wide web of sheet material such as steel, it is common practice to utilize a slitting line. Slitting lines basically consist of an uncoiler from which a web of sheet material is unwound from a coil, a slitting machine which slits the web into a plurality of strips, and a recoiling mechanism which recoils the strips into individual strip coils.
A slitting machine typically includes a pair of arbors which are rotatably supported at each end within a housing. These arbors bear at least one pair of rotary knives, each arbor carrying one mate of each pair. Each rotary knife cooperates with its mate on the opposing arbor to perform the cutting or slitting operation on the web of material as the web is passed between the arbors. The spacing between the arbors is variable so that the rotary knives may be adjusted to be set apart radially or to intermesh to varying degrees. In addition, each rotary knife is positioned upon its arbor to lie in a desired axial relationship with its mate. Once set, this axial relationship must be maintained within a certain tolerance throughout the production run.
The quality of the slit edges produced by a pair of rotary knives is controlled to a great extent by the axial relationship of the mating knives to one another. For slitting applications it is desirable to maintain a nearly constant finite clearance between all pairs of cutters engaged in shearing the master web. As a rule of thumb, for steel sheet, this clearance is usually equal to about ten percent (10%) of the material thickness although for special applications, it may be more or less than this amount. As an example, for a 0.015 inch thick master web, the desired axial relationship might be a clearance of 0.0015 inches. If the clearance becomes too tight, the slit edges might be of acceptable quality but, there is a possibility that mating rotary knives might touch one another. This results in extreme wearing of the cutting edges and subsequent loss of life to the rotary knives. On the other hand, if the clearance becomes too wide, the slit edges produced may have "burrs" or roughness resulting from excessive tearing of the master web. Clearly, both of these conditions are undesirable.
In order to maintain a fixed axial relationship between mating knives, slitting machines built according to the prior art have been designed to attempt to keep both arbors axially fixed so that when rotary knives are mounted upon the arbors, the axial relationship between mating knives will likewise remain fixed. However, in actual practice, arbors can and do shift axially with respect to one another. The consequence of this axial arbor shift is to produce undesirable changes in the axial relationship between mating rotary knives, resulting in excessive knife wear or poor quality, rough edges on the slit strips. Among other factors, this axial shifting may be caused by thermal expansion of one or both arbors due to heat generated by their support bearings, endplay in one or both arbors, or distortion of the base due to variations in the support of the base introduced by the slitting head being moved in and out of the slitting line.
When such a slitting machine is in operation, an inspector must be present to examine the edges of the slit coils for burrs, indicative of a change in the axial relationship of the rotary knives. When such burrs are observed, the slitting line must be shut down while the axial positions of the rotary knives are readjusted on the arbors. This is a difficult, time consuming process. The production down-time involved, the inferior quality slits which may be produced before the problem is remedied and the need for personnel to readjust the knives all impair the productivity of a slitting line.
Accordingly, there is a need for a slitting apparatus and method which provide for restoring mating rotary knives to a desired axial relationship quickly and easily. Furthermore, there is a need for a slitting apparatus and method which provides for monitoring the relative axial positions of the arbors and which responds automatically to reposition the arbors relative to one another as required to maintain each rotary knife in a desired axial relationship with respect to its mate.